1. Field of the Invention
The present invention relates to a resin composition for coating an optical fiber which has excellent rapid curability and excellent storage stability.
2. Description of Related Art
Optical fiber cables have been put to practical use as a high-capacity information transmission medium, with broadband networks utilizing optical fiber cables now being constructed. Optical fibers are roughly classified into those having a double-layered coated structure, comprising a waveguide glass and primary and secondary coating layers made of a radiation (for example, ultraviolet light) curable resin (hereinafter referred to as a resin composition for coating an optical fiber) provided on the waveguide glass, and those having a single-layered coated structure, comprising a waveguide glass and a single coating layer provided on the waveguide glass. Such an optical fiber provided with the coating layer is referred to as a coated optical fiber, which is sometimes provided with a colored layer on the outermost layer for the purpose of identification to give a colored coated optical fiber. Furthermore, the optical fibers may have a unitary structure obtained by arranging plural coated optical fibers or colored coated optical fibers in the form of concentric circles or planes and integrating them using a resin composition for coating an optical fiber. This unitary structure is useful in realizing a high optical fiber density and is now common in optical fiber cables.
Various characteristics such as high-speed processability, or durability and mechanical strength of a film after curing are required of the resin composition for coating an optical fiber to be used as a primary coating material, a secondary coating material and a material for forming an optical fiber unit. Among these characteristics, rapid curability and the storage stability are important as characteristics of the composition before curing. Specifically, required characteristics are as follows.
(1) Rapid Curability
The resin composition shall be sufficiently cured even in high-speed processing (low light energy dose) at a speed equal to or higher than several hundred meters/minute or several thousand meters/minute to obtain a cured article having a Young's modulus suited for use in the optical fiber.
(2) Storage Stability
The resin composition shall cause neither an increase in viscosity nor deposition of solids during storage for a long period or storage at a high temperature and can be used for coating without any problem even after storage under those conditions.
Other characteristics are as follows.
(3) High-Speed Processability
The resin composition shall be uniformly coated on the surface of a coated optical fiber even in high-speed processing.
(4) Durability
The resin composition shall not cause a change in transmission characteristics and mechanical characteristics of an optical fiber unit when exposed to various environments for a long period, and thus exhibits good durability.
(5) Temperature Characteristics
The resin composition shall not cause a change in transmission characteristics and mechanical characteristics of an optical fiber unit even when exposed to a service temperature within a wide range, and thus exhibits good temperature characteristics.
(6) Toughness
The resin composition shall have good balance between Young's modulus, elongation and breaking strength so as not to be damaged even by harsh treatment.
(7) Water Resistance
The resin composition shall not cause deterioration of transmission characteristics for a long period even if a cable sheath is accidentally broken after the optical fiber cable is laid, resulting in permeation of water into the cable.
Among the prior art proposed for the purpose of improving the rapid curability or storage stability, the prior art proposed for the purpose of achieving rapid curability includes Japanese Unexamined Patent Application, First Publication No. Hei 1-190712, Japanese Unexamined Patent Application, First Publication No. Hei 2-248470, Japanese Unexamined Patent Application, First Publication No. Hei 2-34620, and Japanese Unexamined Patent Application, First Publication No. Hei 2-64510. In the resin composition for coating an optical fiber described in these prior art references, however, not only is rapid curability insufficient, but also the storage stability is unsatisfactory. Also Japanese Unexamined Patent Application, First Publication No. Sho 62-215663 discloses a technique relating to a resin composition for coating an optical fiber which is mixed with a sulfur antioxidant for the purpose of achieving satisfactory stability of a coating film over time after curing. Also Japanese Unexamined Patent Application, First Publication No. 2001-199748 describes a technique relating to a resin composition for coating an optical fiber using a radical polymerizable polyurethane compound having a specific structure and also discloses a technique to improve the rapid curability and storage stability. In the techniques described in these patent publications, however, a resin composition has not been obtained for coating an optical fiber capable of sufficiently satisfying the rapid curability and the storage stability of the composition before curing at a technical level required at present.